Methods for increasing the rate of electrochemical deposition

ABSTRACT

A method for electrochemically processing a microfeature workpiece includes contacting the first surface of the microfeature workpiece with a plating electrolyte in a plating chamber, wherein the plating electrolyte includes at least one metal ion, flowing the plating electrolyte from a first plating electrolyte inlet at the first end of the workpiece to a second plating electrolyte outlet at the second end of the workpiece across the center point of the workpiece, and electrochemically depositing the at least one metal ion onto the first surface of the workpiece. Another method for electrochemically processing a microfeature workpiece includes contacting a first surface of the microfeature workpiece with a plating electrolyte having at least one metal ion, heating the second surface of the workpiece using a heating method, and electrochemically depositing the at least one metal ion onto the first surface of the workpiece.

BACKGROUND

In electrochemical deposition, the electroplating rate is controlled by a number of factors, including, for example, electrical current and the concentration of the metal ions to be plated at the location the plating will occur. The concentration of the metal ions to be plated is a gating factor regardless of applied electrical current.

The limiting current density is the maximum current at which the desired reaction will occur before other undesirable impacts take place. Undesired impacts from exceeding the limiting current density include, but are not limited to, dendritic deposits, nodule formation, and the evolution of gas as a result of alternative reactions.

Therefore, a need exists for increasing the availability of reactive species at the location where plating is to occur to increase the electroplating rate. Embodiments of the present disclosure are directed to fulfilling these and other needs.

SUMMARY

The summary is provided to introduce a selection of concepts in a simplified form further described below in the Detailed Description. The summary is not intended to identify key features of the claimed subject matter, nor is the summary intended to be used as an aid in determining the scope of the claimed subject matter.

In accordance with one embodiment of the present disclosure, a method for electrochemically processing a microfeature workpiece, with the workpiece having a first surface and a second surface and a first end and a second end, is provided. The method includes contacting the first surface of the microfeature workpiece with a plating electrolyte in a plating chamber, wherein the plating electrolyte includes at least one metal ion; flowing the plating electrolyte from a first plating electrolyte inlet at the first end of the workpiece to a second plating electrolyte outlet at the second end of the workpiece across the center point of the workpiece; and electrochemically depositing the at least one metal ion onto the first surface of the workpiece.

In accordance with another embodiment of the present disclosure, a method for electrochemically processing a microfeature workpiece, the workpiece having first and second opposing surfaces, is provided. The method includes contacting a first surface of the microfeature workpiece with a plating electrolyte having at least one metal ion; heating the second surface of the workpiece using a heating method; and electrochemically depositing the at least one metal ion onto the first surface of the workpiece.

In accordance with any of the methods described herein, the plating electrolyte may be a catholyte and the plating electrolyte chamber may be a catholyte chamber.

In accordance with any of the methods described herein, the electrolyte may be pumped to the first plating electrolyte inlet from an electrolyte chamber.

In accordance with any of the methods described herein, the electrolyte chamber may include a pressurized plenum region.

In accordance with any of the methods described herein, the electrolyte may have a flow rate greater than 50 mm/sec at the inlet.

In accordance with any of the methods described herein, the electrolyte may have a flow rate greater than 50 mm/sec at the outlet.

In accordance with any of the methods described herein, the electrolyte may have a substantially unidirectional flow pattern from the first end of the workpiece to the second end of the workpiece.

In accordance with any of the methods described herein, the electrolyte may impinge the first surface of the workpiece at an angle in the range of about 5 to about 10 C.

In accordance with any of the methods described herein, the method may further include flowing the plating electrolyte from a plurality of second plating electrolyte inlets to one or more second plating electrolyte outlets in a substantially parallel flow pattern to the flow pattern of the plating electrolyte from the first inlet to the first outlet.

In accordance with any of the methods described herein, the method may further include heating the second surface of the workpiece using a heating method.

In accordance with any of the methods described herein, the heating may be in the range of 90° to 200° C.

In accordance with any of the methods described herein, the heating method may be selected from the group consisting of direct conductive heating, convective heating, ionic heating, and irradiation.

In accordance with any of the methods described herein, direct conductive heating may be selected from the group consisting of direct contact with a heated vacuum chuck and direct contact with a heated pad.

In accordance with any of the methods described herein, convective heating may include a flow of hot air across the second surface of the workpiece.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of the disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1A and 1B are schematics showing interior views of a workpiece in a plating cell having electrolyte flow patterns in accordance with embodiments of the present disclosure;

FIG. 1C is a close-up schematic view of a feature on the workpiece of FIGS. 1A and 1B showing electrolyte flow patterns in the feature;

FIG. 2 is a schematic showing an interior view of a workpiece in a plating cell having electrolyte flow patterns in accordance with another embodiment of the present disclosure;

FIG. 3 is a schematic showing an interior view of a workpiece in a plating cell in accordance with another embodiment of the present disclosure having electrolyte flow patterns;

FIG. 4A is a schematic showing an interior view of a workpiece in a plating cell in accordance with another embodiment of the present disclosure having backside heating and electrolyte flow patterns;

FIG. 4B is a close-up schematic view of a feature on the workpiece of FIGS. 4A and 1B showing a temperature gradient in the feature;

FIG. 5 is a graphical representation of data showing Rotating Disk Electrode (RDE) limiting current for a copper electrolyte bath at varying temperature;

FIG. 6 is a graphical representation of data showing RDE limiting current for a copper electrolyte bath at varying temperature and agitation speed; and

FIG. 7 is a graphical representation of data showing RDE limiting current for a copper electrolyte bath at varying agitation speed.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to systems and methods for increasing the limiting current density (and thereby, the maximum plating rate) on microelectronic workpieces by means of increasing mass transport to the reaction surface. Factors affecting the deposition of metal in a feature include, but are not limited to, current density, concentration of metal ions in the bath, size of the recess opening, the depth of the recess being plated, the types and concentration of bath additives, agitation of the bath, and the temperature of the bath. These factors are described in greater detail below. Methods described herein are directed to process parameters for improving agitation and plating interface temperature.

Embodiments of the present disclosure are directed to methods for processing workpieces, such as semiconductor wafers, devices or processing assemblies for processing workpieces, and methods of processing the same. The terms “workpiece,” “wafer,” and “semiconductor wafer” means any flat media or article, including semiconductor wafers and other substrates or wafers, glass, mask, and optical or memory media, MEMS substrates, or any other workpiece having micro-electric, micro-mechanical, or microelectro-mechanical devices.

Methods described herein are to be used for metal or metal alloy deposition in features of workpieces, including trenches and vias. In one embodiment of the present disclosure, the processes may be used in small features, for example, features having a feature critical dimensions of 1 to 2 microns in width and a plated thickness of 1 to 2 microns in a masking film of 5 to 10 microns in thickness. In another embodiment, the processes may be used in large features, for example, having a depth of up to 250 microns. In another embodiment, the processes may be used in features having an aspect ratio in the range of 1 to 20. However, the processes described herein are applicable to any feature size. The dimension sizes discussed in the present application may be post-etching feature dimensions at the top opening of the feature.

The processes described herein may be applied to various forms of copper, cobalt, nickel, gold, silver, manganese, tin, aluminum, and alloy deposition, for example, in Damascene or packaging applications. Processes described herein may also be modified for metal or metal alloy deposition in high aspect ratio features, for example, vias in through silicon via (TSV) features.

The descriptive terms “micro-feature workpiece” and “workpiece” as used herein may include all structures and layers previously deposited and formed at a given point in the processing, and is not limited to just those structures and layers as depicted in the figures or described in the present application.

Although generally described as metal deposition in the present application, the term “metal” also contemplates metal alloys and co-deposited metals. Such metals, metal alloys, and co-deposited metals may be used to form seed layers or to fully or partially fill the feature. Exemplary co-deposited metals and copper alloys may include, but are not limited to, copper manganese and copper aluminum. As a non-limiting example in co-deposited metals and metal alloys, the alloy composition ratio may be in the range of about 0.5% to about 6% secondary alloy metal.

Referring to FIGS. 1A-1C, a workpiece 102 in an exemplary plating process in accordance with embodiments of the present disclosure is provided. Referring to FIG. 1A, the workpiece 102 includes a substrate 110, an optional barrier layer 112, and a seed layer 114. In an electrochemical deposition chamber, voltage applies cathodic potential on the workpiece 102 with respect to the anode 104.

The conventional fabrication of metal interconnects may include a suitable deposition of a barrier layer 112 on the substrate 110 dielectric material to prevent the diffusion of copper into the dielectric material. Suitable barrier layers include, for example, titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), manganese (Mn), manganese nitride (MnN), etc. Barrier layers are typically used to isolate copper or copper alloys from dielectric material.

A seed layer 114 may be deposited on the barrier layer 112. The seed layer may be deposited using a PVD deposition technique. As another non-limiting example, the seed layer may be a copper alloy seed layer, such as copper manganese, copper cobalt, or copper nickel alloys. The seed layer may also be formed by using other deposition techniques, such as CVD or ALD.

After the seed layer 114 has been deposited, ECD plating may be performed in the electrochemical deposition chamber. Referring to FIG. 1C, features 124 on the first surface 114 of the workpiece 102 may be plated, for example, using acid plating chemistry or alkaline plating chemistry. ECD fill is typically bottom-up gap fill, super-fill, conformal, or super-conformal plating, all having a goal of substantially void free fill.

Electroplating occurs when metal ions in solution combine with electrons at the plated layer interface. Initially, the metal ions deposit on the seed layer, and then later deposit as additional molecules are plated on top of metal previously deposited. As metal ions are deposited, the concentration of the metal ions at the plating interface will be depleted. The concentration of metal ions at the boundary layer is refreshed as metal ions diffuse from the bulk plating electrolyte to the plating interface. Diffusion is driven in part by a concentration gradient. Therefore, by maintaining a high concentration of metal ions in the bulk plating electrolyte, metal ions will diffuse more readily to the plating interface. However, there are physical limits to the amount of metal ions in the electrolyte. In addition, diffusion can be affect by the depth and/or aspect ratio of the feature.

Certain additives (e.g., suppressors, accelerators, and levelers) can help promote the transfer of ionic species to the reaction surface. Additives are broadly described as accelerators, suppressors, and levelers. Although primarily used to modify film morphology during the electroplating process (such as limiting the formation of voids, and determining grain size and film finish), they may also have an impact on limiting current density.

Limiting current density increases linearly with an increase in concentration of metal ions, an increase in the diffusion constant, or a decrease in the diffusion boundary layer. Because the diffusion constant and the concentration of metal ions in solution cannot be extensively manipulated in a plating electrolyte, methods for increasing limiting current density in accordance with embodiments of the present disclosure primarily decrease the diffusion boundary layer. The steady-state diffusion boundary layer is defined by hydrodynamic conditions on the workpiece surface, and can be decreased by increasing electrolyte flow through heat and/or mass transfer.

In accordance with one embodiment of the present disclosure, agitation is used to increase the bulk transport of metal ions to the reaction interface beyond the mass transport achieved by diffusion. Agitation can decrease the boundary layer through which the metal ions diffuse, depending on the type of agitation and the specific workpiece geometry. For example, agitation of a plating electrolyte when depositing metal on an unpatterned workpiece can be effective as decreasing the boundary layer. In contrast, when depositing metal in features having small geometries and/or high aspect ratios, diffusion can be the more effective transport mechanism than agitation. The feature size defines the restriction through which products and reactants pass to exchange from the reaction interface to the bulk plating electrolyte.

Previously designed systems typically use a paddle agitator in close proximity to the workpiece having approximately the same diameter as the workpiece. The paddle is designed to create a displacement of plating electrolyte to move across the workpiece surface. The paddle can be attached to a movement mechanism to provide short rapid strokes back and forth. The velocity of the paddle can be in the range of about 50 to about 500 mm/second with an acceleration of up to 10× the velocity or 50 to 5000 mm/second**2.

The rapid back and forth strokes prevent a directional flow of electrolyte across the surface of the workpiece. Therefore, the paddle tends to act more like a “mixer” than as a means of imparting velocity to the fluid. While mixing helps to promote certain advantages in the electrolyte, such as uniformity in film composition, mixing has limited benefit in promoting an increase in limiting current.

With a paddle system, the inventors found agitation in the feature was significantly less than agitation achieved near the paddle. In addition, constant reversal of the paddle prevented achieving a true unidirectional bulk flow velocity in the plating bath.

In accordance with one embodiment of the present disclosure, referring to FIGS. 1A and 1B, plating electrolyte 106 flows from one or more electrolyte inlets 118 to one or more electrolyte outlets 120 in a substantially unidirectional velocity profile as illustrated in FIG. 1B. The velocity profile is achieved by a high velocity, substantially unidirectional electrolyte flow pattern from one end of the workpiece to the other end of the workpiece. As can be seen in the illustrated embodiment of FIG. 1B, the flow patterns between adjacent inlets and outlets are substantially parallel.

At the electrolyte inlets 118, electrolyte flow 122 is introduced into the plating chamber in close proximity to the leading edge of the workpiece 102. The inlets 118 may have a height approximately equal to the width of the workpiece to be plated, for example, having a height ranging from 0.5 mm to 5 mm, or the width of a chord of the workpiece. The inlets 118 may have a rectangular or other cross-sectional shape, and may also be shaped in such a way as to follow the contour of the leading edge of the workpiece 102 to which the flow is being introduced.

At the electrolyte outlets 120, electrolyte flow 122 is introduced into the plating chamber in close proximity to the edge of the workpiece 102. The outlets 120 may have a height approximately equal to the width of the workpiece to be plated, for example, having a height ranging from 0.5 mm to 5 mm, or the width of a chord of the workpiece. The outlets 120 may have a rectangular or other cross-sectional shape, and may also be shaped in such a way as to follow the contour of the edge of the workpiece 102 over which the flow 122 flows.

In the illustrated embodiment of FIG. 1A, the plating cell includes a permeable membrane 116, such as an ion permeable membrane, to divide the plating electrolyte into an anolyte 108 and a catholyte 106. Therefore, the catholyte 106 flows through the chamber through inlet 118 and outlet 120. The ion permeable membrane 116 is used to separate the anolyte 108 and the catholyte 106 for anolyte 108 to have different chemical characteristics and properties than the catholyte 106.

In other embodiments, the plating cell does not include a membrane to divide the electrolyte into catholyte and anolyte cells (see, e.g., FIG. 3). In the illustrated embodiment of FIG. 3, the electrolyte 306 flows through the chamber through inlets 318 and outlets 320.

Embodiments of the present disclosure are directed to a means for creating a true velocity profile in the flow of the electrolyte 106 on the surface of the workpiece 102, resulting in an increase in mass transport in the features of the workpiece. Referring to FIG. 1C, a close-up view of a feature 124 on the workpiece shows mixing in the feature 124 as a result of the velocity profile of the electrolyte 106 in FIGS. 1A and 1B. In contrast with previously designed paddle agitation, the unidirectional fluid flow described in the present application provides an advantageous effect of enhanced mixing in features.

While a paddle-type agitator can be effective at mixing as a result of eddies generated at the apex of the paddle fins, a paddle-type agitator does not produce fluid motion in the bulk fluid equal to the motion of the agitator. Inertia of the fluid, the need for a gap between the agitator and the plated surface resulting in a “leak path”, and the reversal of the agitator direction does not allow the fluid to be accelerated and achieve the same velocity as the paddle achieves. By contrast, injecting the electrolyte into a constrained space where the plated surface forms one constraining boundary of the fluid path can be configured in such a way for fluid flow to be substantially parallel to the wafer surface and a constant velocity is maintained across the wafer surface. The boundary layer through which metal ions diffuse then becomes a function of the velocity, rather than a function of the agitator motion profile. The entire bulk of the liquid in the confined space is at velocity, and is substantially uniform across the surface.

In accordance with one embodiment of the present disclosure, the velocity of the electrolyte across the surface of the workpiece at the first end of the workpiece may be greater than 50 mm/sec, greater than 100 mm/sec, greater than 150 mm/sec, or greater than 200 mm/sec. Practical velocities of up to 800 mm/sec having been achieved. Therefore, in one embodiment of the present disclosure, the velocity of the electrolyte across the surface of the workpiece at the first end of the workpiece may be in the range of 100 to 800 mm/sec.

The velocity of the electrolyte across the surface of the workpiece at the second end of the workpiece may be in a similar range greater than 50 mm/sec, greater than 100 mm/sec, greater than 150 mm/sec, greater than 200 mm/sec, or in the the range of 100 to 800 mm/sec. Any reduction in velocity is attributable to flow losses at the workpiece periphery.

Systems for achieving such flow velocity may include pumps, pressurization, or vacuum suction. In one embodiment of the present disclosure, the system includes a pump for pumping electrolyte from an electrolyte reservoir to the plating chamber through the inlet 118 and back to the electrolyte reservoir through the outlet 120. In another embodiment of the present disclosure, the system includes a pump for pumping electrolyte from a plenum region.

In addition to enhanced mixing effects in the features, the electrolyte may be circulating at a faster rate and thereby maintaining a more constant metal ion concentration. For example, in a typical system, the catholyte tank is circulating at a rate of about 4 liters/minute, with about one reactor volume exchange about every minute. With an exemplary high velocity stream in accordance with an embodiment of the present disclosure, the catholyte tank may be circulating at a rate of about 18 to 72 liters/minute, therefore exchanging reactor volume faster than previously designed systems.

As seen in the illustrated embodiment of FIG. 2, an exemplary system in accordance with another embodiment of the present disclosure may include one or more nozzles 224 at the electrolyte inlet 218 to direct the electrolyte across the surface of the workpiece 202.

In addition to directionality across the workpiece, the fluid flow of the electrolyte may be in an impinging angle on the surface of the workpiece (see FIG. 2). In one non-limiting example, the impinging angle may be in the range of about 5 to about 10 C.

In addition to enhanced mixing, increasing temperature in a bulk plating electrolyte can have the advantageous effect of helping drive diffusion to the plating surface interface. However, bath heating can have adverse effects on additives within the electrolyte. Therefore, electrolyte heating parameters are generally within controlled limits.

Referring to FIGS. 4A and 4B, another embodiment of the present disclosure is provided. In accordance with this embodiment, the workpiece 402 is subjected to backside heating on the second surface 410 of the workpiece. Backside heating creates a temperature differential between the plating surface at the bottom of the feature 424 of the workpiece T1 (e.g., 65 C) and the field T2 (e.g., 35 C).

As described above, in accordance with one embodiment of the present disclosure, backside heating of the workpiece may be used to further increase electroplating rate. Backside heating may be used in lieu of or in addition to the methods of electrolyte flow described above. In accordance with embodiments of the present disclosure, the backside surface of the workpiece can be heated to increase the bath temperature in the feature, but not the overall bath temperature.

In backside heating, a gradient of heating is achieved in the fluid in the feature, but the bulk of the fluid is maintained at a lower temperature. Therefore, an increase in temperature at the plating surface interface can help drive diffusion of metal ions to the interface without detrimentally increasing the temperature of the bulk fluid.

In one embodiment of the present disclosure, the temperature of the fluid at the bottom of the recess in the feature may be at least about 20° C. warmer than the temperature of the bulk fluid outside the feature. In another embodiment of the present disclosure, the temperature of the fluid at the bottom of the recess in the feature may be at least about 30° C. warmer than the temperature of the bulk fluid outside the feature. In one embodiment of the present disclosure, the temperature of the fluid at the bottom of the recess in the feature may be in the range of about 60° C. to about 90° C. In one embodiment of the present disclosure, the temperature of the bulk fluid outside the feature may be in the range of about 35° C. to about 50° C.

The heating method for the backside of the workpiece may be direct conductive heating, convective heating, ionic heating, or irradiation. Direct conductive heating may be, for example, direct contact with a heated vacuum chuck or direct contact with a heated pad. Convective heating may include, for example, a flow of hot air across the second surface of the workpiece.

In addition to backside heating, cooling can be applied to the bulk electrolyte to prevent bath degradation.

Example 1 RDE Limiting Current Vs. Temperature

The backside heating concept was tested by attaching a patterned silicon workpiece to a heat-sink. A Rotating Disk Electrode (RDE) was immersed in a copper plating bath with approximately 63 g/l (1 M) Cu in solution from copper sulfate. Fluid at 70 C was circulated through the heat sink, and the patterned workpiece was immersed in a plating bath. Various samples were plated at different currents at both ambient and elevated sample temperature. The results in FIG. 1 show an increase in limiting current density with increasing temperature.

At ambient temperature, the onset of nodule formation occurred at a plating rate of approximately 3 to 3.5 microns/minute when plating inside photoresist vias approximately 75×120 microns (diameter vs. depth of photoresist).

Example 2 RDE Limiting Current Vs. Rotational Speed and Temperature

FIG. 2 shows data representing the results of a Rotating Disk Electrode (RDE) experiment in which both temperature and the rotational speed were varied. Tests were run at rotational speeds of 0 RPM, 5 RPM, 25 RPM, and 100 RPM. The results showed slight increases to limiting current density from increasing rotational speed at low RPM values of 5 and 25. The results showed a step-function increase at higher rotational velocity of 100 RPM.

Example 3 RDE Limiting Current Vs. Rotational Speed

FIG. 3 shows data representing the impact of the relative fluid velocity in greater detail. Current “state of the art” parameters for paddle operate at 200 mm/sec and an acceleration velocity of 8000 mm/sec**2. Rotating Disk Electrode (RDE) sample coupons were run with a substantially linear jet stream, and the electroplating rate increased from 3 to 3.5 microns/minute to more than 10 to 15 microns/minute before nodule formation was observed, indicating we had exceeded the limiting current. The increase was achieved by scanning a velocity jet across the surface of the coupon at a flow velocity of ˜183 mm/sec. The jet stream is near the range for paddle velocity, but achieves better results than paddle velocity because the jet stream appears to impart true unidirectional bulk flow velocity to the electroplating bath. A true velocity profile on the wafer surface results in an increase in mass transport even into restrictive features, enabling a significant increase in plating rate.

While illustrative embodiments have been illustrated and described, various changes can be made therein without departing from the spirit and scope of the disclosure. 

The embodiments of the disclosure in which an exclusive property or privilege is claimed are defined as follows:
 1. A method for electrochemically processing a microfeature workpiece, the workpiece having a first surface and a second surface and a first end and a second end, the method comprising: contacting the first surface of the microfeature workpiece with a plating electrolyte in a plating chamber, wherein the plating electrolyte includes at least one metal ion; flowing the plating electrolyte from a first plating electrolyte inlet at the first end of the workpiece to a second plating electrolyte outlet at the second end of the workpiece across the center point of the workpiece; and electrochemically depositing the at least one metal ion onto the first surface of the workpiece.
 2. The method of claim 1, wherein the plating electrolyte is a catholyte and the plating electrolyte chamber is a catholyte chamber.
 3. The method of claim 1, wherein the electrolyte is pumped to the first plating electrolyte inlet from an electrolyte chamber.
 4. The method of claim 3, wherein the electrolyte chamber includes a pressurized plenum region.
 5. The method of claim 1, wherein the electrolyte has a flow rate greater than 50 mm/sec at the inlet.
 6. The method of claim 1, wherein the electrolyte has a flow rate greater than 50 mm/sec at the outlet.
 7. The method of claim 1, wherein the electrolyte has a substantially unidirectional flow pattern from the first end of the workpiece to the second end of the workpiece.
 8. The method of claim 1, wherein the electrolyte impinges the first surface of the workpiece at an angle in the range of about 5° to about 10° C.
 9. The method of claim 1, the method further comprising flowing the plating electrolyte from a plurality of second plating electrolyte inlets to one or more second plating electrolyte outlets in a substantially parallel flow pattern to the flow pattern of the plating electrolyte from the first inlet to the first outlet.
 10. The method of claim 1, further comprising heating the second surface of the workpiece using a heating method.
 11. The method of claim 10, wherein the heating is in the range of 90° to 200° C.
 12. The method of claim 10, wherein the heating method is selected from the group consisting of direct conductive heating, convective heating, ionic heating, and irradiation.
 13. The method of claim 12, wherein direct conductive heating is selected from the group consisting of direct contact with a heated vacuum chuck and direct contact with a heated pad.
 14. The method of claim 12, wherein convective heating includes a flow of hot air across the second surface of the workpiece.
 15. A method for electrochemically processing a microfeature workpiece, the workpiece having first and second opposing surfaces, the method comprising: contacting a first surface of the microfeature workpiece with a plating electrolyte having at least one metal ion; heating the second surface of the workpiece using a heating method; and electrochemically depositing the at least one metal ion onto the first surface of the workpiece.
 16. The method of claim 15, wherein the heating is in the range of 90° to 200° C.
 17. The method of claim 15, wherein the heating method is selected from the group consisting of direct conductive heating, convective heating, ionic heating, and irradiation.
 18. The method of claim 17, wherein direct conductive heating is selected from the group consisting of direct contact with a heated vacuum chuck and direct contact with a heated pad.
 19. The method of claim 17, wherein convective heating includes a flow of hot air across the second surface of the workpiece. 